The present invention relates to detecting a registration mark associated with an object, and more particularly to detecting a registration mark representing a binary code sequence using three-state sensing and matched filtering.
A vast number of applications exist in which it is necessary or desirable to monitor certain parameters related to an object. One common example is the need to monitor the position, velocity, acceleration, etc. of one or more components in a manufacturing process. For instance, in a largely automated process for manufacturing disposable diapers, certain components (e.g., absorbent pads, waist elastic bands, printed graphics, etc.) must be positioned or aligned with respect to other components (e.g., support layers, etc.) in order to produce an acceptable product. To facilitate this process, registration marks are commonly applied to certain components. These registration marks are then detected during the manufacturing process using sensors to determine parameters of interest including, for example, when a component is present at a particular location, where to position a particular component, etc.
Frequently, a registration mark is applied to an object as a single mark which, when detected by a sensor, produces a single pulse at the sensor output. This approach, however, sometimes results in detection errors. For example, noise in the system may falsely trigger the sensor output, or may prevent detection of the registration mark. Additionally, sensors may confuse one registration mark with another similar mark. Regardless of the cause, these detection errors can all lead to errors in the registration process and component positioning during product manufacture. As a result, the quality of the manufactured product may suffer, and the product itself may have to be discarded at a corresponding cost to the manufacturer.
It is also known to use registration marks which represent a specific binary code sequence referred to as a xe2x80x9cperfect word.xe2x80x9d Perfect words are known to exhibit autocorrelation functions with low sidelobes when used in certain radar applications. In one registration system using perfect words, a matched filter compares each bit in a detected sequence of bits with corresponding bits in a perfect word. The filter output is then incremented by one for each matching pair of compared bits. Thus, in the case of a seven bit perfect word, the filter output will have a maximum amplitude of seven upon detecting a sequence of bits which completely matches the perfect word. As best understood, however, no change is made to the filter output in response to a non-matching pair of compared bits. Therefore, if all but one pair of compared bits match, then the filter output will have an amplitude of six (assuming a seven bit perfect word is used). Thus, the difference in the filter output for matching and non-matching inputs may be as small as one, as is the case in the single pulse registration systems described above.
As recognized by the inventors hereof, what is needed is a registration system which produces a filter output having a high amplitude in response to a matching input, and an advantageously lower amplitude, as compared to the prior art, for non-matching inputs.
In order to solve these and other needs in the art, the inventors hereof have succeeded at designing a system and method for detecting a registration mark associated with an object which provides improved discrimination between matching and non-matching inputs. The registration mark preferably represents a binary code sequence of N bits, where N is an integer greater than one. A sensor is provided to sense for the registration mark, and to output a value indicating whether a positive bit, a negative bit, or no bit of the binary code sequence is detected. A sequence of N sensor values are provided to a code matching filter for comparison with the binary code sequence. The code matching filter preferably increases a value of the detection signal for each sensor value which matches its corresponding bit, decreases the value of the detection signal for each sensor value which is complementary to its corresponding bit, and neither increases nor decreases the value of the detection signal in response to a sensor value indicating detection of no bit of the binary code sequence. When the binary code sequence is a Barker code, the code matching filter yields the ideal Barker response whenever the binary code sequence is detected.
According to one aspect of the invention, a method is provided for sensing a registration mark associated with an object, with the registration mark representing a binary code sequence of N bits with each bit having either a first discrete value or a second discrete value, where N is an integer greater than one. The method includes sensing for the registration mark, and generating a sensor value in response to the sensing. The sensor value takes a first state when a bit having the first discrete value is detected, a second state when a bit having the second discrete value is detected, and a third state when no bit having the first discrete value or the second discrete value is detected.
According to another aspect of the invention, a system is provided for detecting a registration mark associated with an object, with the registration mark representing a binary code sequence of N bits with each bit having either a first discrete value or a second discrete value, where N is an integer greater than one. The system includes a sensor for sensing for the registration mark. The sensor has an output which takes a first state when a bit having the first discrete value is detected, a second state when a bit having the second discrete value is detected, and a third state when no bit having the first discrete value or the second discrete value is detected. The system also includes a shift register for sampling the sensor output over time to produce a sequence of sensor values, and a matching filter operatively connected to the shift register for comparing N of the sensor values with corresponding bits of the binary code sequence, and for generating a detection signal in response to the comparing. The matching filter is configured to increase a value of the detection signal for each sensor value which matches its corresponding bit of the binary code sequence, decrease the value of the detection signal for each sensor value which is complementary to its corresponding bit of the binary code sequence, and neither increase nor decrease the value of the detection signal for each sensor value taking the third state. The detection signal indicates detection of the registration mark when the detection signal exceeds a predefined value.
While some of the principal features and advantages of the invention have been described above, a greater and more thorough understanding of the invention may be attained by referring to the drawings and detailed description of preferred embodiments which follow.